1. Field of the Invention
The present invention relates to a piezoelectric vibrator having an airtight terminal and a method for producing the piezoelectric vibrator, as well as an oscillator, an electronic unit and a wave timepiece having a piezoelectric vibrator.
2. Description of the Related Art
Piezoelectric vibrators are indispensable to the production of industrial products such as watches including wave watches/clocks, oscillators, portable units, and electronic units. Piezoelectric vibrators are used in timekeeping sources, timing sources or reference sources for signals. Piezoelectric vibrator packages commonly used include box-shaped ceramic packages and cylinder-type packages provided with leads. In the latter packages, shaped parts are produced, where leads are mounted directly on substrates by means of soldering, packages are molded of resin and electrode terminals using lead frames are provided so that the packages are suitable for machine mounting using automatic mounting machines.
A cylinder type package and a resin molded package will be described below with reference to the drawings. FIG. 19A is a perspective view showing the entire construction of an example of a quartz crystal vibrator for a related-art cylinder type package as a piezoelectric vibrator. FIG. 19B is a cross-sectional view of an airtight terminal portion of the example shown in FIG. 19A.
A vibrating piece 8 composed of a quartz crystal formed tuning fork type is secured to the inner lead 3 of an airtight terminal 1 by means of plating using a metal film and a conductive adhesive, which are not shown. The vibrating piece 8 is also airtight sealed by means of a bottomed cylindrical metallic case 10 close fit thereto to provide a vacuum. Note that in FIG. 19A the case 10 is shown to be a transparent body for the description of the internal construction thereof. As shown in g. FIG. 19B, the airtight terminal 1 is filled with a filler 5, which is used for hermetic sealing in an outer ring called a stem 7. Two parallel leads 2 each composed of a thin solid metal round bar are inserted through and fixed to the filler 5. The surfaces of the lead 2 and the stem 7 are coated with a plating 16a. The inner lead 3 and the vibrating piece 8 shown in FIG. 19A are connected to each other through the plating 16a by locally melting the plating 16a of the surface of the inner lead 3 and securing the lead 3 to a mount pad 9 as a connection region, which is formed at the base of the vibrating piece 8. An excitation electrode, not shown, is formed on the tuning fork arm portion of the vibrating piece 8 and an alternating current is fed to the excitation electrode from the lead 2 of the airtight terminal 1 through the mount pad 9. This causes the vibrating piece 8 to vibrate at a predetermined frequency.
The case 10 is also press fit to the stem 7 so as to cover the vibrating piece 8 along the outside perimeter of the stem 7. This step is called a capping step or sealing step. The case 10 is airtight bonded to the stem 7 by means of cold pressure welding through the plating 16a, a soft metal which the outer ring of the stem 7 is made of. In FIG. 19B, with the filler 5 as a border, the thickness of the plating 16a is exaggerated. Note that in the lead 2, the side which is bonded to the vibrating piece 8 is herein referred to as an inner lead, depicted by a reference numeral 3, while the side which is mounted on a substrate and the like is referred to as an outer lead 4, depicted by a reference numeral 4.
FIG. 20A is a plan view of a resin molded package for a piezoelectric vibrator of a related art. FIG. 20B is a front view of the package shown FIG. 20A. FIG. 20C is a bottom view of the package shown FIG. 20A. FIG. 20D is a cross sectional view of the package shown FIG. 20A, taken along a line D-D. FIG. 20E is a right-hand side view of the package shown in FIG. 20B.
The resin molded package has a construction of a cylinder type package with piezoelectric vibrators 40, which is molded of resin such as epoxy resin, which is a thermosetting resin, and a liquid-crystal polymer, which has thermal plasticity. The use of these resins 30 are determined considering reliability requirements such as the absence of voids, cracks, and peelings at molding, the absence of deformation and cracks in mounting on a substrate in reflow, moisture resistance at high temperatures. Efforts are so made to prevent the outside dimensions of the resin molded package from exceeding the outside dimensions of a ceramic package. In other words, an external electrode terminal 34 is mechanically and electrically connected to the outer lead 4 of the airtight terminal and the welding region 33 by means of spot welding and the like in a space in the resin molded package. The external electrode terminal 34 is also exposed at the bottom via a crank-shaped bent portion 34a. In addition, dummy terminal 35 each having a rise portion 35a are disposed in a resin 30 at intervals at an end opposite to the external electrode terminal 34 of the resin molded package. Therefore, each of the dummy terminals 35 is rigidly mounted on a substrate. The dummy terminal 35 is not electrically connected to the vibrating piece 8 inside the piezoelectric vibrator 40, thus allowing the width of the resin molded package to sufficiently smaller than a ceramic package. Packages molded of resin are in heave usage in portable units, for which high-density mounting is required.
A process for manufacturing a resin molded package will be briefly described below in terms of problems in the order of an airtight terminal production process, a cylinder type package piezoelectric vibrator assembly process, and a resin molded package production process. The present invention is intended to provide a production method where airtight terminals can be caused to flow in a single support material consistently, without replacing the support material for holding airtight terminals in each of these three steps, from the start of the airtight terminal production process to the end of the resin molded package production process. However, a piezoelectric vibrating piece production process will be described later.
[Airtight Terminal Production Process]
A related-art airtight terminal production process will be described below with reference to a production flow chart shown in FIG. 21.
Parts composed of an airtight terminal are one annular stem, two wire-like leads, and one granular filler. Each of these parts is produced in a step thereof before these parts are assembled together. For the annular stem and the lead, a material such as low carbon steel (Fe), alloy of ion and nickel (Fe—Ni), an alloy of ion, nickel, and cobalt (Fe—Ni—Co) is used. For the material of the filler, Soda lime glass and soda barium glass, borosilicate glass and the like are selected.
For the stem, a plate material of any of the materials described above is prepared (step 500) and an outer ring is shaped through a press work (step 502). Pretreatment is then performed using an acid (step 504). Annealing is then performed and preliminary oxidation is performed for improvements in adhesiveness to the filler (step 506). For the filler, borosilicate glass powder is prepared, for example (step 540). The powder is then charged into a mold for shaping and mechanical properties are conditioned (step 542). Sintering is then performed at a high temperature (step 544).
For the lead, a wire rod made of any of the materials described above is prepared (step 570) and cut to pieces of a predetermined length (step 572). Pretreatment is then performing using an acid (step 574). Annealing is then performed and preliminary oxidation is performed for improvements in adhesiveness to the filler (step 576).
A plurality of stems thus prepared, a plurality of fillers thus prepared, and a plurality of leads thus prepared are set in a carbon assembly jig (step 600). Firing is then performed in an electric furnace or the like at a temperature of approximately 1000° C. to rigidly seal the filler to the stem and the lead for an airtight construction (step 610). On completion of the sealing step by firing described above, in the airtight terminal, the metal stem has been filled with the filler, which is used for hermetic sealing, and two parallel leads has been inserted through and fixed to the filler, thus completing a contour thereof.
Airtight terminals are then removed from the carbon assembly jig and each separate airtight terminal is placed in a predetermined net for barrel plating. A plating film is then formed on the outer peripheral surface of the stem and the surface of the lead while a plurality of airtight terminals are agitated in a barrel. Before the plating operation described above, the surface of the filler, the surface of the stem and the surface of the lead are etched and cleaned using an acid. A few μm of base coat plating of an element such as Ni and Cu is applied to each of these surfaces. A finish coat plating with a thickness of 10 to 15 μm is then applied to each of the above surfaces (step 620).
The airtight terminals are completed through the steps described above. A case, which is paired with the airtight terminal, is formed of a nickel silver sheet through a press work for later use. The surface of the case is coated with a Ni plating.
[Piezoelectric Vibrator Assembly Process]
A vibrator assembly process where cylinder type package piezoelectric vibrators are produced using the airtight terminals and cases described above will be described below with reference to a production flow chart shown in FIG. 22. An automated assembly process for such a cylinder type package piezoelectric vibrator is conventionally known.
A plurality of airtight terminals are arranged and mechanically held at constant intervals on each horseshoe-shaped resin molded jig called a pallet and caused to flow through the assembly step described below.
Before arrangement on a pallet, airtight terminals stored after production are first heated sufficiently in a vacuum and baking is perform to remove and release moisture absorbed and gas components contained from member materials (step 700). The plurality of airtight terminals are then mounted and arranged at constant intervals on the pallet using a jig (step 710). FIG. 24 is a pattern diagram showing a state where airtight terminals are arranged on a pallet. In FIG. 24, outer leads 4 of airtight terminals are mechanically held in metal terminals 39 mounted on a pallet 38 and positioned. The metal terminal 39 is a metal fixing fitting.
Referring back to FIG. 22, a vibrating piece 8 is mounted on (connected to) the inner lead 3 of an airtight terminal 1 (step 720). Baking is then performed in a vacuum to alleviate mount distortion (step 730). Then pallet 38 with the airtight terminals thereon is then charged into a vacuum chamber and frequency trimming (fine trimming) is made using a laser or the like while monitoring the frequency for a frequency in a predetermined frequency range (step 740).
Each case 10 is then disposed in positions opposite to each of the airtight terminals arranged on the pallet 38 and each airtight terminal 1 is press fit to and airtight sealed to each case 10 in a vacuum atmosphere (step 750). In the sealing step, each of the airtight terminals is kept sufficiently hot to release moisture absorbed and gas components contained during the assembly step therefrom before capping in each case 10. Capping the airtight terminal in the case provides the contour of a cylinder type package piezoelectric vibrator 40.
After airtight sealing, screening is performed for frequency stabilization at a predetermined temperature in the atmosphere (step 760). An electrical property inspection is then performed in terms of resonance frequency and resonant resistance values and the like. This step is called a sorting step (step 770). The vibrators thus obtained are removed from a pallet 38 using a dedicated jig (step 780).
The step described above provides complete piezoelectric vibrators 40 each having the contour of a cylinder type package. Mass production of piezoelectric vibrators requires a large number of pallets 38 described above, thus requiring large amounts of money for pallet purchase, maintenance, and disposal including metal-holding fixture replacement.
[Resin Shaping Process]
A production process for molding cylinder the type package piezoelectric vibrator 40 of resin, which is produced through the piezoelectric vibrator assembly process described above will be described below with reference to FIGS. 23 and 25. FIG. 23 is a flow chart showing a related-art resin shaping process for molding a cylinder type package piezoelectric vibrator of resin. FIGS. 25A and 25B are pattern diagrams showing piezoelectric vibrators fixed to a related-art lead frame used in the resin shaping process. FIG. 25A is a plan view of a plurality of piezoelectric vibrators fixed to a related-art lead frame. FIG. 25B is a fragmentary enlarged view of one the plurality of piezoelectric vibrators shown in FIG. 25A. A resin shaping process for such a cylinder type package piezoelectric vibrator 40 is known.
According to a related-art production process, a conductive material is selected for a special lead frame 31 and a lead frame 31 is obtained through blanking as shown in FIG. 25A. As shown in FIG. 25B, a bending work is performed on the lead frame 31 to form an external electrode terminal 34 having a bent portion 34a and a dummy terminal 35 having a rise portion 35a is disposed in an opposite position (step 800). Cylinder type package piezoelectric vibrators 40 are fed to and positioned on the lead frame 31 thus prepared (step 810).
Outer leads 4 of the airtight terminals are then shaped and a portion of an extra length is cut off from each of the outer leads (step 820). The external electrode terminal 14 formed in the lead frame 31 and the outer lead 4 of the airtight terminal are then inserted between a top die and bottom die, welding such as resistance welding is performed to connect the external electrode terminal and the outer lead to each other, and an inspection is performed to check if the external electrode terminal and the outer lead are connected to each other (step 830). A spot welding region is depicted by a reference numeral 33. The plurality of piezoelectric vibrators 40 are welded to the lead frame 31 and arranged in two rows.
A resin material is shaped by means of a transfer mold and the contour of a piezoelectric vibrator shaped is formed (step 840). In FIG. 25B, the mold section 32 for resin shaping is indicated by dashed lines. A marking is performed on the surface of a resin shaped 30 by means of printing, a laser and the like (step 850). Any thin burr from molding is then removed from the resin 30 (step 860). The lead frame 31 is charged into a plating bath to coat the external electrode terminal 34 and the dummy terminal 35 with a plating film not shown with a thickness approximately 5 to 15 μm (step 870). In some cases, a plating film is formed on the lead frame 31 in advance. In these cases, the plating step described above will be omitted.
The lead frame 31 is then cut where the external electrode terminal 34 is located and where the dummy terminal 35 is located to obtain individual separate piezoelectric vibrators resin molded (step 880). A sorting is then performed in terms of resonance frequency and resonant resistance values and the like for vibrators (step 890).
The steps described above provide complete resin molded cylinder type package piezoelectric vibrators. The appearance of a completed piezoelectric vibrator described above is shown in FIGS. 20A to 20D, which have been already described above. Therefore, the appearance of the completed piezoelectric vibrator will not be described below.
Problems relating to flow in the three steps described above can be summarized as shown below.
1) In the airtight terminal production process, three parts, that is, the lead, the filler, and the stem, are fired into a one-piece component and individual separate airtight terminals are placed in a predetermined net for plating operations. Conventionally, barrel plating is employed considering mass productivity because a large number of airtight terminals are handled in a single plating secession. However, a plaint operation for small airtight terminals corresponding to small piezoelectric vibrators suffers from a great drop in yield due to short circuits between airtight terminal leads connected together through plating. The smaller airtight terminals are, the conspicuous the above-mentioned drop in yield is, thus raising a big problem that leads to an increase in airtight terminal unit price.
2) In the piezoelectric vibrator assembly process, resin molded pallets are introduced as jigs for assembly flow. Individual separate airtight terminals are fed to after the end of the airtight terminal production process and the arrangement and mounting of these airtight terminals on pallets requires much time and money, thus resulting in a long operation time with a long a lead time.
With a reduction in the size airtight terminals, more lead bends and leads connected together are found in units for parts supply that uses vibrations, such parts feeders, thus resulting in a low level of contour accuracy for the airtight terminals. Contact between airtight terminals causes fine metal particles and these particles on leads and stems as well as vibrating pieces handled during the assembly step result in different vibrating piece weights and therefore a fluctuation in oscillation frequency.
During heating in a vacuum, pallets made of resin also emit gases in the assembly step and the capping step, resulting in a drop in degree of vacuum. Therefore, there is a problem that these gases causes change in resonance frequency and resonant resistance values over time for vibrators after airtight sealing. As described, there is also changing performance, which is a big problem in the aspect of quality.
3) In the resin shaping process, new special dedicated conductive lead frames need to be prepared. It is necessary to feed cylinder type package piezoelectric vibrators to these lead frames, align each of these piezoelectric vibrators with each lead frame with good accuracy, and connect the piezoelectric vibrators and the frames together by means of welding and the like. There is a tendency that there are more failures in vibrator feeding and more welding failures with a reduction in the size of vibrators, thus leading to more inspection in processes and therefore increased production costs.
In addition, providing the external electrode terminal and the dummy terminal necessitates a second plating of terminals, thus leading to a great cause of increased lead time in production.
Solutions to a large number of the foregiong problems in conventional piezoelectric vibrator production methods are essential to improvements in assembly accuracy, quality, and reduction in production costs through a reduced lead time in production to accommodate demands in rapid product size reduction.